C) At extreme compression, atoms are fully ionized, so electrons go free and can no longer exert pressure.
D) The exclusion principle excludes electrons from exerting more pressure than the uncertainty principle would otherwise allow.
E) Electrons are very small and are simply incapable of exerting much pressure.
27) Which of the following statements about degeneracy pressure is not true?
A) Explaining the origin of degeneracy pressure requires both the quantum mechanical exclusion principle and the uncertainty principle.
B) Degeneracy pressure stops the gravitational collapse of white dwarfs.
C) Degeneracy pressure stops the gravitational collapse of ordinary (main-sequence) stars like the Sun.
D) Degeneracy pressure halts the collapse of neutron stars.
E) Degeneracy pressure arises only with fermions (such as electrons, protons, and neutrons) and not with bosons (such as photons or helium nuclei).
28) Which of the following statements about quantum tunneling is true?
A) It is used in modern electronics, and, in fact, our modern-day computers would not work without it.
B) It plays a crucial role in nuclear fusion in the Sun.
C) It allows electrons and other subatomic particles to pass through wall-like energy barriers even when it seems they do not have enough energy to get through the barriers.
D) Although it has been observed to occur, it violates all other known laws of nature, and explaining it therefore represents a major challenge to physicists.
E) A, B, and C are true, but D is not
29) Which of the following best explains the phenomenon of black hole evaporation?
A) Due to high temperatures in the accretion disk around a black hole, material evaporates from the black hole like water evaporating from the ocean.
B) Particles (or antiparticles) are occasionally ejected from within the event horizon, causing the black hole to lose mass.
C) Particles (or antiparticles) are created by a quantum mechanical effect near, but outside, the event horizon of the black hole. The law of conservation of energy maintains that the black hole must lose energy to "pay" for the creation of this mass.
D) Virtual particles created near the black hole are constantly annihilating each other, causing a very high temperature even if the black hole has no accretion disk. This high temperature provides escape velocity for the virtual particles, causing the entire "cloud" of virtual particles to expand away into space.
E) Black hole evaporation is a virtual process, meaning that it has been theorized by astrophysicists but doesn't really occur.
30) Which of the following statements about Hawking radiation is not true, at least according to our current understanding?
A) It was first predicted by Stephen Hawking in the 1970s.
B) It causes black holes to slowly shrink in mass.
C) It has never yet been observed.
D) It represents a strange form of radiation that emerges from within the event horizon of a black hole.
E) It occurs when real particles are created from the gravitational potential energy of a black hole.
S4.2 True/False Questions
1) The electromagnetic force is the force that holds nuclei together.
2) The number of neutrinos in the universe is greater than all the protons, neutrons, and electrons combined.
3) The basic building blocks of matter are protons, neutrons, and electrons.
4) To our current knowledge, antimatter does not really exist; it is purely science fiction.
5) Fermions obey the exclusion principle, but bosons do not.
6) The thermal pressure in an object increases when the temperature increases, but the degeneracy pressure does not.
7) Since it was first predicted in the 1970s, scientists have observed the evaporation of about a dozen black holes.
8) Although we cannot see virtual particles, some experimental evidence suggests that they really exist.
S4.3 Short Answer Questions
1) The strong nuclear force is the force that holds the protons and neutrons in the nucleus together. Based on the fact that atomic nuclei can be stable, briefly explain how you can conclude that the strong force must be even stronger than the electromagnetic force, at least over very short distances.
Answer: An atomic nucleus is made of protons and neutrons. The neutrons have no charge, but protons are positively charged. Thus, if it were just up to the electromagnetic force, a nucleus would fall apart due to the repulsion between the positive protons. Since the strong force is holding the nucleus together despite the electromagnetic repulsion, it must be the stronger (per particle) force within the nucleus. Note that this strength holds only over distances roughly the size of an atomic nucleus. Over larger distances, the strong force cannot be felt at all.
2) All chemical and biological reactions involve the creation and breaking of chemical bonds, which are bonds between the electrons of one atom and the electrons of others. Given this fact, explain why the electromagnetic force governs all chemical and biological reactions. Also explain why the strong force, the weak force, and gravity play no role in these reactions.
Answer: Gravity is far too weak to play a role in creating and breaking bonds between atoms or molecules; in fact, its only role in life is in keeping us "stuck" to the ground. The nuclear forces are of such short range that they have no effects outside the nucleus itself. The only force that remains is the electromagnetic force, which influences interactions between the charged electrons and nuclei. Thus, all events in our ordinary lives–all chemistry and biology–are dominated by the electromagnetic force.
3) The electromagnetic force between two charged particles is much greater than the strength of gravity between them, no matter how far apart they are. Nevertheless, it is gravity, rather than the electromagnetic force, that dominates the universe on large scales. Briefly explain why.
Answer: Despite its far greater strength per particle, the electromagnetic force is unable to attain very large values because it is impossible to accumulate a very large charge. This is because large objects tend to have equal amounts of positive (protons) and negative (electrons) charge, and the electromagnetic force thus "cancels" itself out. Gravity, on the other hand, always attracts: As objects get more and more massive, gravity continues to gain strength. Thus, for very large objects, there will be a great deal of gravitational attraction but virtually no electromagnetic force, since the overall object is neutral.
4) Briefly state the uncertainty principle.
Answer: The more we know about where a particle is located, the less we can know about its momentum, and the more we know about its momentum, the less we can know about its position.
5) What is quantum tunnelling?
Answer: Quantum tunnelling is the name given to the mechanism by which small particles can overcome energy barriers. For example, an electron in a low energy level can occasionally "tunnel" through an energy barrier to a higher energy level. The effect arises because the absolute energy of an object is never precisely known because of the uncertainty principle. The uncertainty, in absolute terms, is very small, but for quantum particles, the relative uncertainty can be significant and occasionally allows a particle to "jump over" an energy barrier.
6) What is "Hawking radiation"?
Answer: The uncertainty principle allows for virtual particles to pop in and out of existence on extremely short timescales anywhere in space. The production of such particles near the event horizon of a black hole can result in one of the pair entering the black hole and the other remaining outside. Without its virtual partner, this particle becomes a "real" part of space (its mass is equivalent to the gravitational energy lost by its other half falling to the center of the black hole). By symmetry we expect equal numbers of matter and anti-matter particles to form around the black hole and these will eventually meet and annihilate each other, producing what is termed Hawking radiation. The creation of mass-energy from a quantum fluctuation comes at the expense of the black hole which (very slowly) evaporates.
7) Briefly explain how antimatter can be produced by scientists.
Answer: Antimatter is produced in particle accelerators. When particles collide and release large amounts of energy in one place, some of the energy spontaneously turns into mass. This mass must contain particles and their antiparticles in equal quantities.
8) Briefly explain why antimatter is difficult to store after it is produced in laboratories.
Answer: When matter and antimatter meet, the result is mutual annihilation. Because everything on Earth is made of ordinary matter, it is difficult to prevent antimatter produced in the lab from contacting matter and quickly becoming annihilated.
9) Process of Science: The idea of quantum tunneling seems very counterintuitive. However, there is considerable evidence to support it. Describe one such line of evidence.
Answer: Answers will vary, but may include discussion of fusion in the Sun, or uses in modern technology such as microchips.
10) Process of Science: Why do scientists believe that black holes can't last forever, and do they have any observational evidence to support this idea?
Answer: Scientists surmise that if a particle-antiparticle pair came into existence near an event horizon and one particle crossed the horizon before they could annihilate, the source of energy for the surviving particle would be the black hole itself. This is called Hawking radiation. The theoretical grounding of this idea is strong, but it has not yet been observed.
11) Process of Science: Why is String Theory not really a theory?
Answer: Most of the predictions of string theory cannot be observed using current particle accelerators such as the Large Hadron Collider. Therefore it is more a hypothesis than a bona fide theory.
12) Process of Science: The "standard model” that describes the building blocks of nature works extremely well and has been most recently verified through the discovery of the Higgs boson. Why, then, do some scientists continue to search for a simpler model?
Answer: This is basically Occam's razor: the standard model has many fundamental particles and some scientists believe that the ultimate description of nature should be simpler with a unified force that explains all particle interactions.
S4.4 Mastering Astronomy Reading Quiz
1) What is the primary topic of study in quantum mechanics?
A) the role played by atoms in astronomy
B) subatomic particles and the forces with which they interact
C) the mechanical movements of the heavens
D) the role of relativity in astronomy
2) What do we mean by the spin of a subatomic particle?
A) It describes how fast the particle is rotating on its axis at a given time—if more torque is applied, the particle spins faster.
B) It is one of the basic properties of any particle, and is a measure of the particle's angular momentum.
C) It is a measure of the particle's electrical charge.
D) The spin of a particle describes how the story of the particle is portrayed to journalists.
C) Quantum ideas were expected to be strange, so the fact that they are strange only lends them support.
D) Actually, very few scientists think the quantum ideas are correct, and most expect the theories of quantum mechanics to be completely replaced in the near future.
2) Which of the following is not one of the basic properties by which we classify a subatomic particle?
B) charge (electrical)
3) The primary scientific purpose of a particle accelerator is to
A) make small particles achieve large accelerations.
B) produce energy for commercial use.
C) create particles and study their behavior.
D) make small particles go very fast
4) Which of the following statements about electrons is not true?
A) In an atom, an electron can have two possible values for its spin, usually called spin up and spin down.
B) Electrons are one of the 6 known types of lepton.
C) They are composed of quarks.
D) Electrons can behave both as particles and as waves.
5) Recall that an up quark (u) has a charge of +2/3 and a down quark (d) has a charge of -1/3. Which of the following quark compositions describes a proton?
6) Suppose that, through the malicious act of an eight-dimensional alien being, the strong force was suddenly turned off throughout the universe. What would happen almost immediately to atoms?
A) Electrons would fall into the nuclei of atoms.
B) The nuclei of atoms would fall apart.
C) Atoms would be immediately ionized.
D) Nothing; atoms would be unaffected by this change.
7) The force of gravity is the only one of the four forces felt on very large size scales. Which of the following best explains why the other forces don't play a major role on large scales?
A) The other three forces are all very short in range, and cannot be felt over large distances.
B) The strong and weak forces act only on very small scales, and large objects tend to be electrically neutral and therefore don't feel an electromagnetic force.
C) Gravity is by far the strongest force, so it simply dominates on large size scales.
D) Effects of the strong and weak forces tend to cancel out effects of the electromagnetic force on large scales.
8) Does the uncertainty principle affect our ability to follow the path of a baseball? Why or why not?
A) No, because the uncertainties in the position and momentum of the baseball are so small in comparison to its size and total momentum that they are unnoticeable.
B) Yes, because we cannot know both where the baseball is and which way it is going at the same time.
C) No, because the exclusion principle says that large objects are excluded from the consequences of the uncertainty principle.
D) No, because the uncertainty principle applies only to electrons.
9) All the following statements are true. Which one can be attributed to the exclusion principle?
A) If we know the location of an electron quite well, we will have little idea where it is going next.
B) If an electron and antielectron meet, they will turn into energy through mutual annihilation.
C) An electron has energy equal to its mass times the speed of light squared.
D) Two electrons with the same spin cannot occupy the same energy level in an atom.
10) What else always happens when an electron is produced from energy in a particle accelerator?
A) The electron quickly becomes part of an atom.
B) The electron releases a tremendous amount of energy.
C) An antielectron (positron) is also produced.
D) The electron begins moving at extremely high speed.
11) Uh-oh, the malicious eight-dimensional alien beings are back. This time, imagine that they suddenly changed nature so that neither the uncertainty principle nor the exclusion principle applied to electrons. What would happen almost immediately to atoms?
A) Atoms would annihilate each other, releasing huge amounts of energy.
B) Atoms would be immediately ionized.
C) Nothing; atoms would be unaffected by this change.
D) Electrons would fall into the nuclei of atoms.
12) Why is there a limit to how much compression can be counterbalanced by electron degeneracy pressure?
A) At extreme compression, the electron speeds approach the speed of light and therefore cannot increase further.
B) At extreme compression, electrons are forced to stop moving, and once stopped there is nothing more they can do.
C) At extreme compression, atoms are fully ionized so that electrons go free and can no longer exert pressure.
D) Because electrons are very small, they are simply incapable of exerting much pressure.
13) Which of the following statements about quantum tunneling is not true?
A) It is used in modern electronics and, in fact, our modern-day computers would not work without it.
B) Although it has been observed to occur, it violates all other known laws of nature and explaining it therefore represents a major challenge to physicists.
C) It plays a crucial role in nuclear fusion in the Sun.
D) It allows electrons and other subatomic particles to pass through wall-like energy barriers even when it seems they do not have enough energy to get through the barriers.
14) Which of the following best explains the hypothesized phenomenon of black hole evaporation?
A) Particles (or anti-particles) are occasionally ejected from within the event horizon, causing the black hole to lose mass.
B) Virtual particles created near the black hole are constantly annihilating each other, causing a very high temperature even if the black hole has no accretion disk. This high temperature provides escape velocity for the virtual particles, causing the entire "cloud" of virtual particles to expand away into space.
C) Particles (or anti-particles) are created by a quantum mechanical effect near, but outside, the event horizon of the black hole. The law of conservation of energy maintains that the black hole must lose energy to "pay" for the creation of this mass.
D) Black hole evaporation is a virtual process, meaning that it has been theorized by astrophysicists, but doesn't really occur.